Characterization of six base pair deletion in the putative HNF1-binding site of human PXR promoter

Pregnane X receptor (PXR) regulates transcription of drug metabolism genes such as CYP3A4 and MDR1. Several species of PXR transcripts have been reported, including hPAR-2 with an extended amino-terminus. Database search identified a 6-bp deletion at the putative HNF1 binding element on the proximal region flanking to the hPAR-2 transcription start site. Aspirin-induced asthma (AIA) is a typical drug-induced phenotype due to aspirin or nonsteroidal antiinflammatory drugs, and these drugs are metabolized by CYP2C9 and UGT1A6, which are regulated by PXR. We examined a possible association between the 6-bp deletion variant and AIA; 129 AIA patients and 117 controls were genotyped, and no allelic association was observed. Characterization of the hPAR-2 promoter revealed that the proximal region of 1.5-kb from the transcription start site conferred a promoter activity and that the 6-bp deletion diminished the activity. These results suggest that the putative HNF1 binding element is essential for the transcriptional activity of hPAR-2 and also, that substantial numbers of Japanese are in a deficient state. Because of the biological significance of the 6-bp deletion of PXR, the variant might potentially associate with as yet unknown phenotype.